The present invention relates to a method for depositing fine-grained, crystalline Al.sub.2 O.sub.3 (alumina) coatings on cutting tools having a body of cemented carbide, cermet, ceramics or high speed steel by means of a Plasma Activated Chemical Vapor Deposition (PACVD) process with the chemical reactants AlCl.sub.3, O.sub.2, H.sub.2 and Ar. The plasma is produced by applying a bipolar pulsed DC voltage on two electrodes or two sets of electrodes. It is possible, with the present method, to deposit at high deposition rate, smooth, high-quality coatings consisting of either single phase gamma-Al.sub.2 O.sub.3 or of a mixture of gamma- and alpha-Al.sub.2 O.sub.3 phases. The coatings have a good wear resistance when applied on cutting tools. With the present method, crystalline Al.sub.2 O.sub.3 coatings can be obtained at deposition temperatures as low as 500.degree. C. The present method, bipolar pulsed DC voltage PACVD, can also successfully be used for the deposition of non-insulating coatings such as TiC, TiN, TiCN and TiAIN, or other carbides and/or nitrides with the metal element chosen from Nb, Hf, V, Ta, Mo, Zr, Cr, and W.
It is well known that for cemented carbide cutting tools used in metal machining, the wear resistance of the tool can be considerably increased by applying thin, hard surface layers of metal oxides, carbides or nitrides with the metal either selected from the transition metals from the groups IV, V and VI of the Periodic Table or from silicon, boron and aluminum. The coating thickness usually varies between 1 and 15 .mu.m and the most widespread techniques for depositing such coatings are PVD (Physical Vapor Deposition) and CVD (Chemical Vapor Deposition). Cemented carbide cutting tools coated with alumina layers have been commercially available for over two decades. The CVD technique usually employed involves the deposition of material from a reactive gas atmosphere of AlCl.sub.3, CO.sub.2 and H.sub.2, on a substrate surface held at elevated temperatures around 1000.degree.C.
Al.sub.2 O.sub.3 crystallizes into several different phases such as .alpha. (alpha), .kappa. (kappa) and .chi. (chi) called the ".alpha.-series" with hcp (hexagonal close packing) stacking of the oxygen atoms, and into .gamma. (gamma), .theta. (theta), .eta. (eta) and .delta. (delta) called the ".gamma.-series" with fcc (face centered cubic) stacking of the oxygen atoms. The most often occurring Al.sub.2 O.sub.3 -phases in coatings deposited with CVD methods on cemented carbides at temperatures 1000.degree.-1050.degree. C., are the stable alpha and the metastable kappa phases, however, the metastable theta phase has occasionally been observed. These Al.sub.2 O.sub.3 coatings of the .alpha.-, .kappa.- and/or .theta.-phase are fully crystalline with a grain size in the range 0.5-5 .mu.m and the coatings have a well-faceted grain structure.
The inherently high deposition temperature of about 1000.degree. C. renders the total stress in CVD Al.sub.2 O.sub.3 coatings on cemented carbide substrates to be tensile. Hence, the total stress is dominated by thermal stresses caused by the difference in the thermal expansion coefficients between the cemented carbide substrate and the coating. The tensile stresses may exceed the rupture limit of Al.sub.2 O.sub.3 and cause the coating to crack extensively and a network of cooling cracks will be generated over the entire Al.sub.2 O.sub.3 layer.
Alternative deposition methods for the production of refractory coatings such as alumina, are desirable, particularly methods capable of operating at lower substrate temperatures which therefore not only allow more temperature sensitive tool substrates to be coated, such as high speed steel, but also eliminates cooling cracks caused by the thermal stresses in the coating. A refractory coating deposited at lower temperatures would also result in a finer grain structure and possibly, a higher hardness of the coating.
Potential low temperature deposition technologies for the production of refractory coatings such as TiC, TiN and Al.sub.2 O.sub.3 on cutting tools are PVD (Physical Vapor Deposition) and PACVD (Plasma Activated CVD). However, certain problems arise when employing these plasma-based techniques for the deposition of highly insulating layers such as Al.sub.2 O.sub.3. The alumina layer grows not only on the substrates but equally on all surfaces in the vicinity of the plasma as well as on the cathodes/electrodes. Furthermore, these insulating layers will become charged which may cause electrical breakdown and arcing. This latter phenomenon naturally effects both the growth rate and the quality of the coating in a detrimental way.
One solution to the above problems has been the invention of the bipolar pulsed DMS technique (Dual Magnetron Sputtering) which is disclosed in DD 252 205 and U.S. Pat. No. 5,698,314. In the bipolar dual magnetron system, the two magnetrons alternately act as an anode and as a cathode and hence, preserve the magnetron targets in a metallic state over long process times. At high enough frequencies, possible surface charging on the insulating layers will be suppressed and the otherwise troublesome phenomenon of arcing will be limited. According to U.S. Pat. No. 5,698,314, the DMS sputtering technique is capable of depositing and producing high-quality, well-adherent, crystalline .alpha.-Al.sub.2 O.sub.3 thin films at substrate temperatures less than 800.degree. C.
The PVD techniques in general have, due to the low process pressure, the disadvantage of being so-called "line of sight" methods, i.e., only surfaces facing the ion source will be coated. This disadvantage can to a certain extent be compensated for by rotating the substrates during the deposition.
A prior art Plasma Assisted CVD method for the deposition of Al.sub.2 O.sub.3 layers of the .alpha.- and/or .gamma.-Al.sub.2 O.sub.3 polymorphs at substrate temperatures between 450.degree. and 700.degree. C. is disclosed in U.S. Pat. Nos. 5,516,588 and 5,587,233. This PACVD process includes the reaction between an Al-halogenide, e.g., AlCl.sub.3, and CO.sub.2, H.sub.2 and Ar in a plasma generated by applying a unipolar pulsed DC voltage on the substrate body connected as a cathode which means that the substrate is constantly held at a negative potential. A disadvantage with the DC voltage generation of plasmas, including the unipolar pulsed DC voltage technique, is that the surface charging on the nonconducting layers cannot totally be suppressed. Specifically, the charging is most severe on sharp corners and along edges of the substrates resulting in a significant decrease in the layer thickness and also in the quality of the coating.
In more general terms, the fact that the insulating alumina layer grows not only on the substrates but equally on all surfaces in the vicinity, the plasma as well as on the electrodes, will negatively influence the stability of the plasma and the entire deposition process may eventually end in the extinction of the discharge.
Still another factor which effects the growth rate of the coating is that the deposition process will be interrupted every time the unipolar pulsed DC voltage is at zero potential. In U.S. Pat. No. 5,093,151, the unipolar pulsed DC voltage being used to produce the plasma is deliberately not allowed to attain zero potential between the pulses but is held at a residual potential which is always larger than the lowest ionization potential of any of the element in the reaction mixture H, H.sub.2, Ar, O, O.sub.2 and AlCl.sub.3. The ratio of the residual voltage and the maximum voltage of the pulse is said to be 0.02-0.5. By not allowing the voltage to attain zero potential may have a preferable effect on the deposition rate but simultaneously results in a more severe charge built-up on non-conducting surfaces.